As well known in the art, a crane is a machine or a machine apparatus which is designed to hoist an object using a driving force and moves in a horizontal direction and is designed to operate a winding and unwinding operation for lifting or lowering a hoisting object and a horizontal direction operation and a swing operation for horizontally moving the lifted object. So, the hoist works properly in a 3D space with the help of the above combined works,
The crane consists of a pair of running rails, and a girder which is installed on the running rail and moves along the same, and a hoist which is installed on the girder and shifts along the same.
A running wheel is installed at both ends of the girder and is mounted on the running rail. A running motor is installed at the running wheel. When the running motor is driven, the running wheel rotates and allows the girder to move along the running rail.
The hoist installed at the girder comprises a hoist frame which supports the entire structure of the hoist, a cross running motor which is installed at the hoist frame and is mounted on the girder and shifts the hoist frame along the girder, a wire drum which is rotatably installed at the hoist frame and winds or unwinds a wire rope, a winding motor which is connected with the wire drum for driving the same, and a hook which is connected with a lower end of the wire rope and moves up or down as the wire rope is wound or unwound from the wire drum.
When a certain weight object is hung by the crane, the girder is moved along the X axis and Y axis by driving the running motor. The hoist installed on the girder is moved along the X axis and Y axis by driving the cross running motor, so that the weight object is positioned while matching with the coordinates X and Y of the hoist. The hook is lowered down by driving the winding motor of the hoist and is connected with the weight object, so that it is ready to move the weight object. So, finally it is possible to move the weight object in the reverse procedure of the above operation.
FIGS. 1 and 2 are schematic front views illustrating a conventional hoist for describing the problems encountered in the conventional art. FIGS. 3 and 4 are a schematic perspective view and a side cross sectional view another example of a conventional hoist for a low space crane. The problems of the conventional hoist will be described with reference to the above drawings.
FIGS. 1 and 2 are schematic front views illustrating a conventional hoist for describing the problems encountered in the conventional art. As shown therein, the hoist 3 of FIG. 1 is installed so that the hoist frame 4 is mounted on the upper side of the girder 2 of the running rail 1. Since part of the hoist 3 is exposed by the height h in the upper direction of the girder 2, a certain space at the upper side of the girder 2 is further needed. So, the installation height of the crane is lowered by the above space height.
The hoist 13 of FIG. 2 is installed at both ends of the girder 12 of the upper side of the running rail 11. So, almost parts of the hoist 13 are installed lower than the lower side of the girder 12. As compared to the hoist 3 of FIG. 1, since the conventional hoist 13 does not need the height h which is protruded from the upper side of the girder 12, it is possible to increase the entire height of the crane, but almost parts of the hoist 13 are installed below the lower side of the girder 12, so that the vertical transfer distance of the hook 14 decreases by the lowered height h′. So, the vertical transfer distance for hanging and lifting the weight object decreases.
So as to overcome the problems of FIGS. 1 and 2, the applicant of the present invention disclosed a Korean patent application number 10-2006-6576 filed on Jan. 21, 2006 (and published on Nov. 30, 2006 as Korean patent publication number KR-100654786) in which as shown in FIGS. 3 and 4, the installation height of the hoist 30 and the installation height of the girder 21 are overlapped, so that the total installation heights are minimized. The hoist 30 of FIGS. 3 and 4 is considered to be prior art for the purposes of the present application. The girder 21 has octagonal cross sections, and a horizontal contact surface 21a is formed at the upper center of the girder 21. A slant surface 21b, which is slanted downward, is formed at both sides of the contact surface 21a. A vertical surface 21c is formed at both sides of the slant surface 21b. According to the conventional girder 21, when a weight object is lifted, the weight of the lifting object is property supported by the contact surface 21a, namely, the upper center portion of the girder 21.
The hoist 30 mounted on the girder 21 contacts with the upper center of the girder 21, namely, the contact surface of the girder 21, so that the weight point P′ properly works.
The low space crane hoist 30 includes a hoist frame 31 for supporting the entire structure with respect to the girder 21. The hoist frame 31 comprises a support frame 32 which is positioned at the center and supports the entire structure, a front frame 33 of which an upper end is fixed at both front sides of the support frame 32, and which is extended in a downward direction, a rear frame 34 of which an upper end is fixed at both rear sides of the support frame 32, and which is formed in a 90 degree angled shape, and a drum cover 35 which is fixed at the both front frames 33 and has a wire drum 43.
A plurality of elements are installed at the hoist frame 31 for lifting and moving a weight object. A lower support wheel part 36 mounted on the girder 21 is installed at both ends of the support frame 32, and a cross running motor 38 is connected with the load support wheel part 36 for thereby driving the load support roller 37 of the load support wheel part 36.
A pair of front upper support wheel parts 39 are installed at the upper side of the front frame 33 of the hoist frame 31 and are supported at the front upper side of the girder 21. A pair of front lower support wheel parts 40 are installed at the lower side of the front frame 33 and are supported at the front lower side of the girder 21. A rear support wheel part 42 is installed at the connection frame 41 connected with a pair of the rear frames 34 and is supported at the rear side of the girder 21.
A wire drum 43 is installed at the drum cover 35, and a winding motor 45 is connected with the wire drum 43. As the winding motor 45 operates, the wire drum 43 rotates for thereby winding or unwinding the wire rope 44, so that the hook 46 connected with the wire rope 44 moves up and down.
According to the conventional low space crane hoist 30, the load support wheel part 36, which supports the entire loads of the hoist 30 of the hoist frame 31 and the weight object, is installed at the support frame 32 of the center portion of the hoist frame 31. The girder 21 is designed to properly support the hoist 30.
The conventional low space crane hoist has the following problems.
First, since the weight of the weight object of the hoist 30 is concentrated at the upper center of the girder 21, the girder 21 may be buckled. According to the conventional hoist 30, the support frame 32, which supports the structure of the hoist frame 31, is positioned at the center of the hoist frame 31, and the load support wheel part 36 is installed at the support frame 32, and the load point P of the hoist frame 31 corresponds to the portion, where the support frame 32 and the load support wheel part 36 are positioned, namely, the center portion of the hoist frame 31. So, the load point P′ is positioned at the center portion of the girder 21 which supports the load support wheel part 36 as the hoist frame 31 is mounted on the girder 21.
The hoist 30, which allows the load point P′ is positioned at the center of the girder 21, causes the girder 21 to buckle owing to the following reasons.
The load point P′ is applied to the contact surface 21a of the upper center of the girder 21 on which the low space crane hoist 30 is mounted, and two slant surfaces 21b support the same. The above two slant surfaces 21b are supported by means of the two vertical surfaces 21c. The load of the weight object is not directly transferred to the vertical surfaces 21c which support almost parts of the load, but is applied through the two slant surface 21 in slant directions. In a state that a plate is arranged in a vertical direction, when loads are applied to the plate at a slant angle, it bears a relatively larger load. However, when the load is applied in a slant direction, it may be buckled.
So, when a certain load of the weight load is applied to the contact surface 21a of the girder 21 of FIGS. 3 and 4, the load is transferred to the vertical surface 21c through the slant surface 21b, and the load applied to the two vertical surfaces 21c is not transferred in the vertical direction along the cross section center of the vertical surface 21c, but is transferred along the slant surface 21b at a slant angle, so that the buckling phenomenon of the girder 21 occurs a lot.
Second, when the weight object is hung by the hook 46 of the hoist 30 and is moved, the weight object may swing. The swing load is transferred to the hoist frame 31 via the wire rope 44, and the impact load transferred to the hoist frame 31 is directly transferred to the load support wheel part 36. The impact load transferred to the load support wheel part 36 causes the load support roller 37 to noncontinuously contact with the girder 21, so that the driving force transferred from the cross running motor 38 to the load support roller 37 is instantly prevented from being applied to the girder 21.
So, since the driving force transferred to the cross running motor 38, the load support roller 37 and the girder 21 is instantly disconnected, the driving efficiency of the hoist 30 decreases, so that the nonuniform transfer speed occurs.
Third, each element belonging to the hoist frame 31, namely, the support frame 32, a pair of the front frames 33 and a pair of the rear frames 34 are formed in a cubic pipe shape or a hollow rectangular pipe shape. So, the entire size and weight of the hoist frame 31 increases owing to a relatively larger size of each element belonging to the hoist frame 31. The hoisting capacity decreases by the increased weight of the hoist.
Fourth, two front upper support wheel parts 39 are installed at a pair of the front frames 33 for stably supporting the hoist frame 31 on the girder 21, and two front lower support wheel parts 40 are installed at the lower side of the front frame 33, and two rear support wheel parts 42 are installed at a pair of the rear frames 34, so that six support wheel parts are totally needed.
So, since six support wheel parts are needed and installed, the productivity of the hoist 30 decreases. As the number of support wheel parts increases, the hoist frame 31 and the girder 21 can be more stably supported. However, in this case, since the impact load transferred to the hoist frame 31 is directly transferred to the support wheel part, the life of each support wheel part is shortened.